The water contact angle measurements Figure 3 (b) confirm that
the natural nutshells (NNSs) are hydrophilic in nature regardless of any
treatments. This indicates that water can easily enter their micro and
nanochannels which is essential to the generation of streaming potential
or streaming current. Figure 3 (b) also demonstrates the
enhanced hydrophilicity of the nutshells after being treated with
acetone and ethanol solutions and subsequently dried in a nitrogen
environment. This treatment process reduces the nonproductive adsorption
between lignin and cellulose proteins, and hence the hemicellulose and
lignin components are partially weakened and
removed.[42] Furthermore, the execution of plasma
treatment caused an incredibly hydrophilic surface which is confirmed by
the water contact angle measurement (S-V1).
Therefore, more hydrophilic functional groups are exposed. Concurrently,
densities showed small declines (Figure-S 2 ), means this
treatment slightly enhanced porosity. It should be noted that this
treatment didn’t remove the lignin contents confirmed by the FT-IR
spectra Figure 3 (a) and CLSM images Figure 3 (d). To
enhance the surface areas, additional treatments are required to remove
the lignin and break the lignin and cellulose links. However, the
elimination of lignin will cause a structural collapse that will
compromise the stability of the device.[43] Thus,
the minor enhancement of the surface areas and porosity by these
treatments ensures the long-term stability of the device. Out of these
four NSs examined, the Almond Shell (AS) has the highest surface areas
and porosity, while the pecan shell (PS) has the lowest surface areas
and porosity, although the variations are quite small. Walnut shell
(WS), on the other hand, demonstrates a well-balanced mix of surface
areas, porosity, zeta potentials, and superior mechanical stability.[44] Therefore, this study focuses on the
functionally graded WS structure for most analyses.
Figure 3 (c) shows how color perception changes while exposed
to the different treatments of WS. After being freeze-dried for 12 h,
the color of the WS samples transforms from a dark orange tone to a pale
orange.